Optical glass

ABSTRACT

A thorium oxide-free B 2  O 3  --La 2  O 3  --Gd 2  O 3  --Ta 2  O 5  system optical glass is improved in stability to devitrification and in optical constants by introducing at least one of a divalent metal oxide such as ZnO, CdO, PbO, BaO, SrO, CaO, and MgO into the optical glass composition or replacing Ta 2  O 5  of the optical glass composition with a divalent metal oxide. If desired, the glass may be free of CdO.

This is a continuation of application Ser. No. 274,381, filed July 24, 1972, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thorium oxide-free and, if desired, a cadmium oxide-free optical glass having improved optical constants n_(d) and ν_(d). In particular, the invention relates to thorium oxide-free optical glass having optical constants of 1.59-1.91 for n_(d) and 62-19 for ν_(d).

2. Description of the Prior Art

Various high refractive and low dispersive optical glasses have hitherto been reported, but many of them contain thorium oxide which is poisonous to humans because of its radioactivity. However, when they are free of thorium oxide, the content of a single component such as La₂ O₃ becomes markedly larger and thus the tendency toward devitrification is increased, which makes the mass production of such glasses difficult.

It was successful to obtain a thorium oxide-free and stable optical glass by replacing a part or the entire amount of La₂ O₃ with Gd₂ O₃. That is, because the properties of Gd₂ O₃ are similar to those of La₂ O₃, a part of or the entire amount of La₂ O₃ of an optical glass can be replaced with Gd₂ O₃ without changing the optical properties. Thus, by preventing an increase in the tendency toward devitrification caused by the use of a large amount of La₂ O₃, the optical glass can be both stabilized with respect to devitrification and the vitrification range can be enlarged. It, in fact, becomes possible to obtain optical glasses possessing improved optical constants that are not present in conventional optical glasses.

As a glass composition containing Gd₂ O₃, a B₂ O₃ --La₂ O₃ -- Gd₂ O₃ --Ta₂ O₅ system is already known from the specification of French Pat. No. 1,529,337, and an optical glass possessing a large vitrification range and improved optical constants which are not present in conventional optical glasses can be obtained. However, such a glass is lacking in satisfactory stability to devitrification, and even if the most stable composition disclosed in the above patent is employed, the optical glass is not so stable that it can be produced on an industrial scale.

Furthermore, if the content of B₂ O₃ in the B₂ O₃ --La₂ O₃ -- Gd₂ O₃ --Ta₂ O₅ system glass is high, the glass tends to have phase separation. Accordingly, an optical glass having a refractive index n_(d) of lower than 1.76 has a strong tendency towards devitrification and it has hitherto been difficult to obtain a high refractive and low dispersive optical glass in the refractive index range.

SUMMARY OF THE INVENTION

The inventors have succeeded, in one embodiment of this invention, in increasing the stability of the B₂ O₃ --La₂ O₃ --Gd₂ O₃ -- Ta₂ O₅ system optical glass by introducing a divalent metal oxide or oxides (RO, wherein R is a divalent metal) to the optical glass system, and thus an optical glass possessing excellent optical constants never before obtained in the B₂ O₃ --La₂ O₃ --Gd₂ O₃ -- Ta₂ O₅ system glass has been obtained.

Moreover, the inventors have also succeeded, in another embodiment of this invention, in increasing the stability of the B₂ O₃ --La₂ O₃ --Gd₂ O₃ --Ta₂ O₅ system optical glass by replacing the Ta₂ O₅ with divalent metal oxides (RO). That is, by providing the specific B₂ O₃ --La₂ O₃ --Gd₂ O₃ --RO system optical glass according to one embodiment of this invention, a high refractive and low dispersion optical glass possessing a high stability to devitrification can be obtained in a range of a comparatively low refractive index.

Furthermore, a conventional optical glass possessing the optical constants of 1.68-1.86 for n_(d) and 53-27 for ν_(d) contains CdO as a necessary component and the powder of the CdO-containing glass formed in the polishing of the glass is poisonous to humans, but in both embodiments of this invention it is possible to provide a CdO-free optical glass possessing the aforesaid ranges of optical constants.

DETAILED DESCRIPTION OF THE INVENTION

That is, according to one embodiment of this invention, there is provided a ThO₂ -free and, if desired, a CdO-free optical glass having a composition of 18 to 60% by weight B₂ O₃ + SiO₂, the content of the SiO₂ being, however, 0 to 23% by weight; 0 to 50% by weight La₂ O₃, 2 to 50% by weight Gd₂ O₃, 2 to 30 % by weight Ta₂ O₅, 0 to 50% by weight ZnO, 0 to 53% by weight, CdO, 0 to 60% by weight PbO, 0 to 50% by weight BaO, 0 to 45% by weight SrO, 0 to 30% by weight CaO, 0 to 20% by weight MgO, the content of the ZnO + CdO + PbO + BaO + SrO+ CaO + MgO being 2 to 60% by weight, 0 to 16% by weight Al₂ O₃, 0 to 9% by weight ZrO₂, 0 to 10% by weight TiO₂, 0 to 26% by weight Nb₂ O₅, 0 to 15% by weight WO₃, and 0 to 18% by weight Y₂ O₃. The optical glass of this invention is quite stable to devitrification, has optical constants of 1.62-1.91 for n_(d) and 59-27 for ν_(d), and can be easily produced on a large scale.

Now, as is well-known, B₂ O₃ and SiO₂ are glass forming oxides and if the sum of these oxides is higher than 60%, phase separation occurs, while if the sum is less than 18%, devitrification of the optical glass occurs. Furthermore, the SiO₂ contributes to increasing the viscosity of the molten glass and to prevent the occurrence of devitrification of the glass, but if the content of the SiO₂ is higher than 23%, the melting point of the glass batch is increased and further the tendency toward devitrification is increased.

La₂ O₃ and Gd₂ O₃ are used for increasing the refractive index and the Abbe number, but if the content of La₂ O₃ and Gd₂ O₃ is higher than 50%, a great tendency toward devitrification occurs, and if the content is less than 2%, the optical constants as described above cannot be obtained.

To obtain a particularly stable optical glass, it is desirable that the Gd₂ O₃ be present in almost the same amount (by weight %) as the amount of La₂ O₃.

Ta₂ O₅ has a glass-forming function and is profitable in a composition containing a low proportion of the above-described glass forming oxides, and is necessary for maintaining a high refractive index in the optical glass composition in the embodiment of this invention described above. However, if the content of Ta₂ O₅ is too large, the devitrification tendency increases. Thus, the content thereof is less than 30%. In the embodiment of this invention, however, the desired optical constants cannot be obtained if the Ta₂ O₅ content is less than 2%.

The divalent metal oxides are necessary components for stabilizing the optical glass toward devitrification, but if the content of ZnO is higher than 50%, CdO is higher than 53%, PbO is higher than 60%, BaO is higher than 50%, SrO is higher than 45%, CaO is higher than 30%, and MgO is higher than 20%, the glass composition is not vitrified. Furthermore, if the sum of more than two of these divalent metal oxides is above 60%, the composition is not also vitrified, but if the content of at least one of these oxides is lower than 2%, sufficient stability to devitrification cannot be obtained. In order to obtain a high refractive and low dispersive optical glass having particularly excellent optical properties, the presence of ZnO and CdO is effective.

Al₂ O₃, ZrO₂, TiO₂, Nb₂ O₅, and WO₃, which are all subsidiary oxides, are added mainly to prevent the devitrification of the glass, to increase the viscosity for improving the glass-forming property, to increase the chemical durability of the glass, and to control the optical constants of the glass. However, if the content of Al₂ O₃, Nb₂ O₅, and WO₃ is higher than 16%, 26%, and 15%, respectively, the tendency toward devitrification is increased. Also, if the content of ZrO₂ is higher than 9%, insoluble precipitates are formed when the feed materials are melted, and if the content of TiO₂ is higher than 10%, the color of the glass product is yellow brown, which makes the use of the glass as an optical glass difficult. Because Y₂ O₃ has almost the same properties as those of Gd₂ O₃, almost the same refractive index and the Abbe number are obtained as those obtained by the addition of Gd₂ O₃. Therefore, a part of the Gd₂ O₃ may be replaced with comparatively inexpensive Y₂ O₃. The co-presence of Gd₂ O₃ and Y₂ O₃ contributes to an increase in the stability of the glass to devitrification, but if the content is above 18%, the glass shows a strong tendency toward devitrifivation.

In particular, to obtain an optical glass having optical constants of 1.76 - 1.86 for N_(d) and 51 - 35 for ν_(d), an optical glass having a composition of 0 to 8% by weight SiO₂, 21 to 30% by weight B₂ O₃, 0 to 3% by weight Al₂ O₃, 25 to 40% by weight La₂ O₃, 2 to 25% by weight Gd₂ O₃, 2 to 20% by weight ZnO, the content of the CdO + PbO + BaO + SrO + CaO + MgO being 0 to 10% by weight, 0 to 6% by weight ZrO₂, 2 to 15% by weight Ta₂ O₅, O to 12% by weight Nb₂ O₅, 0 to 6% by weight WO₃, O to 3% by weight TiO₂, and 0 to 10% by weight Y₂ O₃, is preferred.

According to another embodiment of the present invention, there is provided a ThO₂ -free and, if desired, a CdO-free optical glass showing a high stability to devitrification and having a composition (by percent by weight) of 24 to 70% of B₂ O₃ + SiO₂, the content of the SiO₂ being, however, 0 to 25%, 1 to 50% La₂ O₃, 2 to 50% Gd₂ O₃, 0 to 56% ZnO, 0 to 61% CdO, 0 to 66% PbO, 0 to 50% BaO, 0 to 40% SrO, 0 to 25% CaO, 0 to 20% MgO, the content of the ZnO + CdO + PbO + BaO + SrO + CaO + MgO being 2 to 66%, 0 to 20% Al₂ O₃, 0 to 10% ZrO₂, 0 to 10% TiO₂, 0 to 15% Nb₂ O₅, 0 to 10% WO₃, and 0 to 20% Y₂ O₃. The optional glass of this invention has optical constants of 1.59 - 1.86 for n_(d) and 62 - 19 for ν_(d).

Now, if the content of the sum of B₂ O₃ and SiO₂ is higher than 70%, phase separation occurs, while if the content is lower than 24%, devitrification is formed. On the other hand, as described above the SiO₂ is used to increase the visocity of the molten glass and to prevent the occurrence of devitrification, but if the content of the SiO₂ is higher than 25%, the melting temperature of the glass components is increased and the tendency towards devitrification is increased.

Also, if each of the La₂ O₃ and the Gd₂ O₃ is higher than 50%, the glass tends to be devitrified greatly, while if the content of the La₂ O₃ is lower than 1% and that of the Gd₂ O₃ is lower than 2%, the desired optical constants cannot be obtained. As is the case of the above-described first embodiment of this invention, it is desirable for obtaining a particularly stable optical glass that the La₂ O₃ be present in almost the same amount as the amount of the Gd₂ O₃.

If the content of the ZnO is higher than 56%, the CdO is higher than 61%, the PbO is higher than 66%, the BaO is higher than 50%, the SrO is higher than 40%, the CaO is higher than than 25%, and the MgO is higher than 20%, the glass composition is not vitrified. Furthermore, if the sum of more than two oxides is over 66%, the glass composition is not vitrified, while if the content of at least one of these oxides is lower than 2%, sufficient stability to the devitrification cannot be obtained.

Moreover, if the Al₂ O₃ is higher than 20%, the Nb₂ O₅ is higher than 15%, and the WO₃ is higher than 10%, the tendency toward devitrification is increased. Also, if the content of the ZrO₂ is higher than 10%, insoluble precipitates are formed at melting, and if the TiO₂ is higher than 10%, the glass is yellow brown and thus it cannot be used as optical glass. The co-presence of Gd₂ O₃ and Y₂ O₃ contributes towards increasing the stability to devitrification of glass as described above, but if the content is above 20%, the glass tends to show strong devitrification.

To obtain an optical glass having the optical constants of 1.71 - 1.78 for n_(d) and 55 - 48 for ν_(d), an optical glass having a composition of 1 to 10% by weight SiO₂, 28 to 37% by weight B₂ O₃, 0 to 3% by weight Al₂ O₃, 27 to 45% by weight La₂ O₃, 2 to 15% by weight Gd₂ O₃, 0 to 10% by weight Y₂ O₃, 2 to 15% by weight ZnO, the content of the CdO + PbO + BaO + SrO + CaO + MgO being 0 to 10% by weight, the content of the Ta₂ O₅ + Nb₂ O₅ + WO₃ + TiO₂ being 0 to 3% by weight, and 0 to 8% by weight ZrO₂, is preferred and furthermore, in this case, the co-presence of Gd₂ O₃ and Y₂ O₃ contributes to increase the stability to devitrification of the glass as described above.

To obtain an optical glass having the optical constants of 1.68 - 1.73 for n_(d) and 57 - 52 for ν_(d), an optical glass having a composition of 1 to 15% by weight SiO₂, 25 to 41% by weight B₂ O₃, 0 to 8% by weight Al₂ O₃, 10 to 41% by weight La₂ O₃, 2 to 25% weight Gd₂ O₃, 0 to 10% by weight Y₂ O₃, the content of the CdO + PbO + BaO + SrO + MgO being 0 to 10% by weight, the content of the Ta₂ O₅ + Nb₂ O₅ + WO₃ + TiO₂ being 0 to 3% by weight, and 0 to 5% by weight ZrO₂, is preferred.

Furthermore, to obtain an optical glass having the optical constants of 1.62 - 1.69 for n_(d) and 62 - 55 for ν_(d), an optical glass having a composition of 1 to 20% by weight SiO₂, 25 to 41% by weight B₂ O₃, 0 to 10% by weight Al₂ O₃, 2 to 25% by weight La₂ O₃, 2 to 15% by weight Gd₂ O₃, 0 to 10% by weight Y₂ O₃, the content of the BaO + SrO + CaO + MgO being 10 to 50% by weight, the content of the ZnO, + CaO + PbO being 0 to 10% by weight, the content of the Ta₂ O₅ + Nb₂ O₅ + WO₃ + TiO₂ being 0 to 3% by weight, and 0 to 8% by weight ZrO₂, is preferred.

Now, examples of the first embodiment of the optical glass of this invention are shown in Table 1 (each component is shown in weight percent) together with the optical constants:

                                      Table 1                                      __________________________________________________________________________     No.  B.sub.2 O.sub.3                                                                     La.sub.2 O.sub.3                                                                    Gd.sub.2 O.sub.3                                                                    Ta.sub.2 O.sub.5                                                                    ZnO  CdO  PbO  BaO  SrO  CaO  MgO                     __________________________________________________________________________     1    18.0 32.5 12.0 17.5 5.0                                                   2    21.0 32.0 20.0 25.0      2.0                                              3    22.0 18.0 20.0 20.0           15.0                                        4    22.0 28.0 15.0 5.0  10.0 5.0                                              5    22.0 30.0 20.0 10.0 10.0                                                  6    23.0 35.0 5.0  5.0  5.0                                                   7    24.0 26.0 10.0 30.0      10.0                                             8    24.0 10.0 10.0 5.0                 35.0                                   9    10.0 10.0 5.0  3.0                 47.0                                   10   25.0 35.0 20.0 5.0  5.0                                                   11   25.0 30.0 20.0 10.0 10.0                                                  12   25.0 40.0 5.0  2.5  5.0                                                   13   25.0 29.0 5.0  2.5  5.0                                                   14   25.0 35.0 23.0 15.0 2.0                                                   15   25.0 30.0 2.0  20.0 5.0                                                   16   24.0 11.0 15.0 20.0      30.0                                             17   29.0 24.0 10.0 10.0 27.0                                                  18   30.0      50.0 5.0  15.0                                                  19   30.0 38.0 20.0 2.0       2.0                                              20   30.0 8.0  10.0 2.0  50.0                                                  21   30.0 5.0  10.0 2.0       53.0                                             22   30.0 3.0  4.0  3.0            60.0                                        23   30.0 5.0  10.0 5.0                 50.0                                   24   30.0 5.0  30.0 5.0       30.0                                             25   30.0 30.0 20.0 2.5  10.0                                                  26   35.0 10.0 5.0  5.0                      45.0                              27   35.0 34.0 15.0 2.0  3.0  2.0                                              28   35.0 50.0 2.0  8.0  5.0                                                   29   35.0 5.0  15.0 5.0            40.0                                        30   40.0 13.0 2.0  15.0                          30.0                         31   40.0 15.0 20.0 5.0                      20.0                              32   60.0      5.0  10.0                          25.0                         33   45.0 20.0 10.0 5.0                                20.0                    34   25.0 40.0 5.0  20.0 5.0                                                   No.  SiO.sub.2                                                                           Al.sub.2 O.sub.3                                                                    Y.sub.2 O.sub.3                                                                     TiO.sub.2                                                                           ZrO.sub.2                                                                          Nb.sub.2 O.sub.5                                                                    WO.sub.3                                                                            n.sub.d                                                                             ν.sub.d                         __________________________________________________________________________     1                   2.5  5.0 5.0  2.57 1.8849                                                                         34.2                                    2                                      1.8449                                                                              41.7                               3                   5.0                1.8672                                                                              29.8                               4                                 15.0 1.8119                                                                              40.6                               5    8.0                               1.7583                                                                              47.6                               6                   5.0  5.0 17.0      1.9035                                                                              31.8                               7                                      1.8272                                                                              40.1                               8         16.0                         1.6890                                                                              52.4                               9    23.0 2.0                          1.6675                                                                              52.2                               10                  10.0               1.8566                                                                              33.0                               11        5.0                          1.7743                                                                              47.1                               12                  2.5  5.0 15.0      1.8718                                                                              35.6                               13                  2.5  5.0 26.0      1.8969                                                                              29.9                               14                                     1.8135                                                                              46.0                               15             18.0                    1.8083                                                                              44.6                               16                                     1.8139                                                                              40.3                               17                                     1.7583                                                                              46.9                               18                                     1.7475                                                                              50.2                               19                           8.0       1.7973                                                                              44.6                               20                                     1.7160                                                                              47.7                               21                                     1.7607                                                                              44.0                               22                                     1.8191                                                                              27.2                               23                                     1.6894                                                                              52.8                               24                                     1.7608                                                                              46.5                               25                       5.0      2.5  1.7720                                                                              48.0                               26                                     1.6667                                                                              55.3                               27                       9.0           1.7570                                                                              49.8                               28                                     1.74568                                                                             50.0                               29                                     1.7638                                                                              36.0                               30                                     1.6815                                                                              52.4                               31                                     1.6880                                                                              55.4                               32                                     1.6238                                                                              55.4                               33                                     1.6499                                                                              56.1                               34             5.0                     1.8152                                                                              44.3                               __________________________________________________________________________

In particular, examples of the first embodiment of this invention applied to the B₂ O₃ --La₂ O₃ --Gd₂ O₃ --Ta₂ O₅ --ZnO type optical glass and B₂ O₃ --La₂ O₃ --Gd₂ O₃ --Ta₂ O₅ --CdO system optical glass are shown in Table 2 together with the optical constants and the liquidus temperatures. From the values of the liquidus temperatures, it will be understood that the stabilities of the optical glasses of this invention to devitrification are markedly higher than that of a conventional B₂ O₃ --La₂ O₃ --Gd₂ O₃ --Ta₂ O₅ system optical glass.

                                      Table 2                                      __________________________________________________________________________     No.  B.sub.2 O.sub.3                                                                    La.sub.2 O.sub.3                                                                    Gd.sub.2 O.sub.3                                                                    Ta.sub.2 O.sub.5                                                                    ZnO  CdO n.sub.d                                                                             ν.sub.d                                                                         Liquidus                                                                       Temp.                                                                          (°C)                          __________________________________________________________________________     1    22.0                                                                               33.0 10.0 20.0 15.0     1.8219                                                                              42.8                                                                               1055                                 2    25.0                                                                               35.0 15.0 15.0 10.0     1.8048                                                                              45.4                                                                               1045                                 3    30.0                                                                               30.0 20.0 15.0 5.0      1.7763                                                                              47.1                                                                               1035                                 4    30.0                                                                               30.0 20.0 5.0  15.0     1.7618                                                                              49.4                                                                                985                                 5    35.0                                                                               20.0 25.0 5.0  15.0     1.7284                                                                              50.9                                                                                960                                 6    25.0                                                                               25.0 20.0 10.0 20.0     1.7865                                                                              46.4                                                                               1015                                 7    25.0                                                                               30.0 15.0 20.0 5.0   5.0                                                                               1.8106                                                                              43.6                                                                               1085                                 8    30.0                                                                               40.0 10.0 10.0      10.0                                                                               1.7836                                                                              47.4                                                                               1050                                 9    30.0                                                                               20.0 10.0 5.0       35.0                                                                               1.7689                                                                              45.5                                                                                940                                 10   30.0                                                                               30.0 20.0 10.0      10.0                                                                               1.7788                                                                              47.5                                                                               1025                                 11   40.0                                                                               10.0 10.0 5.0       35.0                                                                               1.7069                                                                              49.1                                                                                930                                 __________________________________________________________________________

Also, examples of applying the invention to the B₂ O₃ -- La₂ O₃ --Gd₂ O₃ --Ta₂ O₅ --ZnO system glass and the B₂ O₃ --La₂ O₃ --Gd₂ O₃ --Ta₂ O₅ -- PbO type optical glass are shown in Table 3, which shows the glass could be stably melted without using CdO in the optical constant ranges of 1.72 - 1.85 for n_(d) and 50 - 27 for νd, the range of which CdO is required in a conventional glass.

                                      Table 3                                      __________________________________________________________________________     B.sub.2 O.sub.3                                                                     La.sub.2 O.sub.3                                                                    Gd.sub.2 O.sub.3                                                                    Ta.sub.2 O.sub.5                                                                    ZnO PbO n.sub.d                                                                             ν.sub.d                                                                         Liquidus                                                                       Temp.                                                                          (°C)                               __________________________________________________________________________     1 25.0                                                                               5.0 20.0 10.0 40.0    1.7585                                                                              45.4                                                                               1045                                      2 30.0                                                                              25.0 10.0 5.0  30.0    1.7463                                                                              48.5                                                                               930                                       3 30.0                                                                              20.0 10.0 5.0      35.0                                                                               1.8031                                                                              36.0                                                                               960                                       4 30.0                                                                              30.0 10.0 15.0     15.0                                                                               1.7934                                                                              40.8                                                                               1050                                      5 35.0                                                                               5.0  5.0 5.0      50.0                                                                               1.7727                                                                              31.6                                                                               900                                       6 40.0                                                                              10.0 10.0 5.0      35.0                                                                               1.7298                                                                              40.0                                                                               985                                       __________________________________________________________________________

Examples of the B₂ O₃ --La₂ O₃ --Gd₂ O₃ --Ta₂ O₅ -alkaline earth metal oxide-type optical glass are shown in Table 4, which shows that the high refractive and low dispersive optical glass of lower than 1.76 for n_(d), which could not be obtained in a conventional B₂ O₃ --La₂ O₃ --Gd₂ O₃ --Ta₂ O₅ system glass because of the increase in the devitrification tendency, could be melted stably.

                                      Table 4                                      __________________________________________________________________________     B.sub.2 O.sub.3                                                                         La.sub.2 O.sub.3                                                                    Gd.sub.2 O.sub.3                                                                    Ta.sub.2 O.sub.5                                                                    BaO SrO CaO MgO n.sub.d                                                                             ν.sub.d                                                                         Liquidus                                                                       Temp.                                                                          (°C)                   __________________________________________________________________________     1    30.0                                                                               10.0 10.0  5.0 45.0            1.6981                                                                              52.6                                                                                965                          2    30.0                                                                               30.0 10.0 15.0 15.0            1.7558                                                                              48.0                                                                               1060                          3    40.0                                                                               20.0 10.0 10.0 20.0            1.6937                                                                              53.5                                                                                965                          4    45.0                                                                                5.0 10.0  5.0 35.0            1.6488                                                                              57.7                                                                                920                          5    30.0                                                                               20.0 10.0 10.0     30.0        1.7188                                                                              51.0                                                                               1055                          6    45.0                                                                               10.0 10.0 10.0     25.0        1.6595                                                                              55.7                                                                                900                          7    40.0                                                                               30.0 10.0  5.0         15.0                                                                               1.7117                                                                             53.4 1010                              8    55.0                                                                               10.0  5.0 10.0         20.0    1.6207                                                                              56.3                                                                                910                          9    30.0                                                                               40.0 10.0 15.0              5.0                                                                               1.7741                                                                              47.7                                                                               1020                          10   45.0                                                                               20.0 10.0 10.0             15.0                                                                               1.6658                                                                              54.2                                                                               1020                          __________________________________________________________________________

Then, examples of the second embodiment of this invention are shown in the following table, in which each component is shown in weight percent.

                                      Table 5                                      __________________________________________________________________________     No.  B.sub.2 O.sub.3                                                                    La.sub.2 O.sub.3                                                                    Gd.sub.2 O.sub.3                                                                    ZnO CdO PbO BaO SrO CaO                                                                               MgO                                  __________________________________________________________________________     1    24.0                                                                               15.0 11.0     50.0                                                    2    25.0                                                                               10.0 15.0     50.0                                                    3    30.0                                                                                2.0  2.0         66.0                                                4    30.0                                                                               10.0  5.0         45.0                                                5    30.0                                                                               40.0 10.0      5.0                                                    6    35.0                                                                               40.0 20.0  5.0                                                        7    35.0                                                                               45.0 10.0     10.0                                                    8    35.0                                                                                2.0  2.0     61.0                                                    9    35.0                                                                                5.0 10.0             50.0                                            10   30.0                                                                               15.0 30.0     25.0                                                    11   35.0                                                                               35.0 10.0 10.0                                                        12   35.0                                                                               25.0  5.0 15.0                                                        13   35.0                                                                               38.0 25.0      2.0                                                    14   35.0                                                                               10.0 10.0             45.0                                            15   40.0                                                                               30.0  5.0                 20.0                                        16   20.0                                                                               10.0 10.0             35.0                                            17   40.0                                                                               50.0  5.0                          5.0                                18   40.0                                                                                2.0  2.0 56.0                                                        19   45.0                                                                               25.0 10.0                         20.0                                20   45.0                                                                                5.0 10.0                 40.0                                        21   40.0                                                                               25.0  5.0                 20.0                                        22   40.0                                                                               20.0  5.0                 30.0                                        23   40.0                                                                               35.0 10.0                      5.0                                    24   44.0                                                                               37.0  4.0             10.0                                            25   60.0                                                                               10.0  5.0                     25.0                                    26   70.0                                                                                2.0  3.0                     25.0                                    No.  SiO.sub.2                                                                          Al.sub.2 O.sub.3                                                                     Y.sub.2 O.sub.3                                                                    TiO.sub.2                                                                           ZrO.sub.2                                                                          Nb.sub.2 O.sub.5                                                                    WO.sub.3                                                                           n.sub.d                                                                             ν.sub.d                           __________________________________________________________________________     1                                    1.7954                                                                              42.3                                 2                                    1.7868                                                                              42.5                                 3                                    1.8135                                                                              25.9                                 4                  10.0              1.8586                                                                              19.7                                 5                           15.0     1.8159                                                                              40.1                                 6                                    1.7448                                                                              53.2                                 7                                    1.7516                                                                              52.1                                 8                                    1.7154                                                                              47.2                                 9                                    1.6684                                                                              56.5                                 10                                   1.7629                                                                              48.6                                 11                      10.0         1.7530                                                                              50.3                                 12             20.0                  1.7247                                                                              52.7                                 13                                   1.7466                                                                              53.3                                 14   25.0                                                                                2.0                        1.6660                                                                              52.8                                 15                  5.0              1.7184                                                                              49.1                                 16       20.0                     5.0                                                                               1.6880                                                                              50.8                                 17                                   1.7168                                                                              54.8                                 18                                   1.6679                                                                              52.0                                 19                                   1.6508                                                                              57.3                                 20                                   1.6304                                                                              60.2                                 21                               10.0                                                                               1.6869                                                                              52.4                                 22    5.0                            1.6529                                                                              58.5                                 23             10.0                  1.7106                                                                              55.4                                 24        1.0            4.0         1.6873                                                                              55.0                                 25                                   1.6290                                                                              61.1                                 26                                   1.5976                                                                              62.8                                 __________________________________________________________________________

Examples of applying the second embodiment of this invention to the B₂ O₃ --La₂ O₃ --Gd₂ O₃ --ZnO system optical glass and the B₂ O₃ --La₂ O₃ --Gd₂ O₃ --CdO system optical glass are shown in Table 6, from which it will be understood that the glasses had excellent optical properties and a greatly increased stability to devitrification in comparison with the conventional B₂ O₃ --La₂ O₃ -- ZnO (or CdO) system glass. In particular, the high refractive and low dispersive optical glass of lower than 1.76 for n_(d), which could not be obtained in the conventional B₂ O₃ --La₂ O₃ --Gd₂ O₃ --Ta₂ O₅ system caused by the increased tendency of devitrification, could be easily melted.

                                      Table 6                                      __________________________________________________________________________     B.sub.2 O.sub.3                                                                     La.sub.2 O.sub.3                                                                    Gd.sub.2 O.sub.3                                                                    ZnO CdO n.sub.d                                                                             ν.sub.d                                                                         Liquidus                                                                       Temperature                                                                    (°C)                                    __________________________________________________________________________     1 30.0                                                                              25.0 20.0 25.0    1.7453                                                                              50.1                                                                                980                                           2 35.0                                                                              40.0 10.0 15.0    1.7372                                                                              52.3                                                                               1035                                           3 35.0                                                                              35.0 20.0  5.0                                                                                5.0                                                                               1.7425                                                                              52.5                                                                               1055                                           4 40.0                                                                              35.0  5.0 20.0    1.7066                                                                              53.2                                                                                990                                           5 30.0                                                                              20.0 20.0     30.0                                                                               1.7669                                                                              47.7                                                                                955                                           6 35.0                                                                              30.0 10.0     25.0                                                                               1.7430                                                                              50.3                                                                               1015                                           7 35.0                                                                              25.0 30.0     10.0                                                                               1.7402                                                                              52.6                                                                               1060                                           __________________________________________________________________________

Also, examples of the B₂ O₃ --La₂ O₃ --Gd₂ O₃ --ZnO system optical glass and the B₂ O₃ --La₂ O₃ --Gd₂ O₃ --PbO system optical glass of this invention are shown in the following table, which shows that the optical glass containing no CdO could be stably melted in the optical constant ranges of 1.68-1.86 for n_(d) and 53-30 for ν_(d), which required CdO as a necessary component in conventional glasses.

                                      Table 7                                      __________________________________________________________________________     B.sub.2 O.sub.3                                                                     La.sub.2 O.sub.3                                                                    Gd.sub.2 O.sub.3                                                                    ZnO PbO n.sub.d                                                                             ν.sub.d                                                                         Liquidus                                                                       Temperature                                                                    (°C)                                    __________________________________________________________________________     1 30.0                                                                              15.0  5.0 50.0    1.7172                                                                              48.2                                                                                905                                           2 35.0                                                                              30.0 10.0 25.0    1.7238                                                                              51.6                                                                                965                                           3 40.0                                                                              25.0  5.0 30.0    1.6940                                                                              52.6                                                                                930                                           4 30.0                                                                              10.0 10.0     50.0                                                                               1.8106                                                                              32.6                                                                                950                                           5 35.0                                                                              15.0 30.0     20.0                                                                               1.7490                                                                              44.4                                                                               1055                                           6 35.0                                                                              15.0 10.0     40.0                                                                               1.7677                                                                              37.3                                                                               1000                                           7 40.0                                                                              10.0 10.0     40.0                                                                               1.7308                                                                              39.3                                                                               1000                                           8 45.0                                                                               5.0 10.0     40.0                                                                               1.6953                                                                              41.0                                                                                965                                           __________________________________________________________________________

Examples of the B₂ O₃ --La₂ O₃ --Gd₂ O₃ -alkaline earth metal oxide system optical glass of this invention are shown in Table 8, which shows that the optical glasses of this invention had excellent optical properties and a markedly increased stability to devitrification in comparison with the conventional B₂ O₃ --La₂ O₃ alkaline earth metal oxide system optical glasses.

                                      Table 8                                      __________________________________________________________________________     B.sub.2 O.sub.3                                                                       La.sub.2 O.sub.3                                                                    Gd.sub.2 O.sub.3                                                                    BaO SrO CaO MgO n.sub.d                                                                             ν.sub.d                                                                         Liquidus                                                                       Temp.                                                                          (°C)                          __________________________________________________________________________     1  35.0                                                                               10.0 10.0 45.0            1.6764                                                                              56.1                                                                                980                                 2  40.0                                                                               20.0 30.0 10.0            1.6988                                                                              56.0                                                                               1050                                 3  35.0                                                                               15.0 20.0     30.0        1.6905                                                                              55.7                                                                               1005                                 4  40.0                                                                               30.0 20.0     10.0        1.7023                                                                              55.9                                                                               1025                                 5  45.0                                                                               5.0  20.0     30.0        1.6504                                                                              59.5                                                                                980                                 6  40.0                                                                               35.0 20.0          5.0    1.7146                                                                              55.3                                                                               1065                                 7  45.0                                                                               15.0 20.0         20.0    1.6765                                                                              57.3                                                                               1025                                 8  50.0                                                                               30.0 10.0         10.0    1.6590                                                                              57.9                                                                                995                                 9  35.0                                                                               25.0 30.0             10.0                                                                               1.7125                                                                              54.4                                                                               1025                                 10 40.0                                                                               30.0 20.0             10.0                                                                               1.6939                                                                              55.5                                                                               1050                                 11 45.0                                                                               30.0 10.0             15.0                                                                               1.6638                                                                              57.0                                                                               1055                                 __________________________________________________________________________

In the above examples, the liquidus phase temperature value was obtained by maintaining the glass sample in a conventional inclined devitrification furnace for 30 minutes.

The above-described optical glasses are obtained by melting in a platinum crucible at 1300°-1400° C a mixture of boric acid, lanthanum oxide, gadolinium oxide, tantalum pentoxide, zinc white, cadmium carbonate, litharge, barium carbonate, strontium nitrate, powdered silica, zirconium oxide, aluminum hydroxide, niobium oxide, titanium oxide, tungsten oxide, yttrium oxide, etc., or a mixture of boric acid, lanthanum oxide, gadlinium oxide, zinc white, cadmium carbonate, litharge, barium carbonate, strontium nitrate, calcium carbonate, powdered silica, zirconium oxide, aluminum hydroxide, niobium oxide, tungsten oxide, titanium oxide, yttrium oxide, etc., homogenizing the molten mixture by stirring, defoaming the melt, casting the molten glass in a metallic mold pre-heated to a suitable temperature, and annealing it.

In view of the foregoing, the inventors' inventive glasses can include the following preferred component systems:

A thorium oxide-free glass having a liquidus temperature not greater than 1085° C and having optical constants of 1.62-1.91 for n_(d) and 59-27 for νd consisting essentially of a composition, in weight percent of 18 to 60% B₂ O₃ + Sio₂, a quantity of B₂ O₃ being present and the content of the SiO₂ being 0 to 23%, 0 to 50 % La₂ O₃, 2 to 50% Gd₂ O₃, 2 to 30% Ta₂ O₅, 0 to 50% ZnO, 0 to 53% CdO, 0 to 60% PbO, 0 to 50% BaO, 0 to 45% SrO, 0 to 30% CaO, 0 to 20% MgO, the sum of the ZnO + CdO + PbO + BaO + SrO + CaO + MgO being 2 to 60%, 0 to 16% Al₂ O₃, 0 to 9% ZrO₂, 0 to 10% TiO₂, 0 to 26% Nb₂ O₅, 0 to 15% WO₃, and 0 to 18% Y₂ O₃ ;

a thorium oxide-free optical glass wherein the glass has the composition, in weight percent, of 18 to 40% B₂ O₃, 10 to 45% La₂ O₃, 2 to 35% Gd₂ O₃, 2 to 25% Ta₂ O₅, 0 to 25% ZnO, 0 to 30% CdO, the sum of the ZnO + CdO being 2 to 30%;

a thorium oxide-free optical glass wherein the glass has a composition, in weither percent, of 22 to 40% B₂ O₃, 0 to 30% La₂ O₃, 2 to 30% Gd₂ O₃, 2 to 30% Ta₂ O₅, 25 to 40% ZnO, 0 to 20% PbO, the sum of the ZnO + PbO being 25 to 45%;

a thorium oxide-free optical glass wherein the glass has a composition, in weight percent, of 25 to 45% B₂ O₃, 0 to 40% La₂ O₃, 2 to 30% Gd₂ O₃, 2 to 20% Ta₂ O₅, 0 to 40% BaO, 0 to 35% SrO, 0 to 20% CaO, 0 to 20% MgO, the sum of the BaO + SrO + CaO + MgO being 2 to 40%;

a thorium oxide-free optical glass having optical constants of 1.77- 1.85 for n_(d) and 51-40 for νd consisting essentially of a composition, in essentially of a composition, in weight percent, of 0 to 8% SiO₂, 21 to 30% B₂ O₃, 0 to 3% Al₂ O₃, 25 to 40% La₂ O₃, 2 to 25% Gd₂ O₃, 2 to 20% NnO, the sum of the CdO + PbO + CaO + BaO + SrO + CaO + MgO being 0 to 10%, 0 to 6% ZrO₂, 2 to 15% Ta₂ O₅, 0 to 12% Nb₂ O₅, 0 to 6% WO₃, 0 to 3% TiO₂ and 0 to 10 Y₂ O₃ ;

a thorium oxide-free optical glass having optical constants of 1.59-1.86 for n_(d) and 62-19 for ν d consisting essentially of a composition, in weight percent, of 24 to 70% B₂ O₃ + SiO₂, a quantity of B₂ O₃ being present and the content of said SiO₂ being 0 to 25%, 1 to 50% La₂ O₃, 2 to 50% Gd₂ O₃, 0 to 56% ZnO, 0 to 61% CdO, 0 to 66% PbO, 0 to 50% BaO, 0 to 40% SrO, 0 to 25% CaO, 0 to 20% MgO, the sum of the ZnO + CdO + PbO + BaO + SrO + CaO + MgO being 2 to 66%, 0 to 20% Al₂ O.sub. 3, 0 to 10% ZrO₂, 0 to 10% TiO₂, 0 to 15% Nb₂ O₅, 0 to 10% WO₃, and 0 to 20% Y₂ O₃ ;

a thorium oxide-free optical glass wherein the glass has a composition, in weight percent, of 24 to 45% B₂ O₃, 1 to 45% La₂ O₃, 2 to 30% Gd₂ O₃, 0 to 25% ZnO, 0 to 30% CdO, the sum of the ZnO + CdO being 2 to 30%;

a thorium oxide-free optical glass wherein the glass has a composition, in weight percent, of 25 to 45% B₂ O₃, 1 to 35% La₂ O₃, 2 to 25% Gd₂ O₃, 25 to 40% ZnO, 0 to 20% PbO, the sum of the ZnO .sup.αPbO being 25 -45%;

a thorium oxide-free optical glass wherein the glass has the composition, in weight percent, of 30 to 50% B₂ O₃, 1 to 45% La₂ O₃, 2 to 30% Gd₂ O₃, 0 to 40% BaO, 0 to 35% SrO, 0 to 25% CaO, 0 to 20% MgO, the sum of the BaO + SrO + CaO + MgO being 2 to 40%;

a thorium oxide-free optical glass wherein the glass has the composition, in weight percent, of 1 to 10% SiO₂, 28 to 37% B₂ O₃, 0 to 3% Al₂ O₃, 27 to 45% La₂ O₃, 2 to 15% Gd₂ O₃, 0 to 10% Y₂ O₃, 2 to 15% ZnO, the sum of the CdO + PbO + SrO + CaO + MgO being 0 to 10%, the sum of the Nb₂ O₅ + WO₃ + TiO₂ being 0 to 3%, and 0 to 8% ZrO₂ ;

a thorium oxide-free optical glass as in the preceding glass wherein the Y₂ O₃ is 2 to 10%;

a thorium oxide-free optical glass wherein the glass has the composition, in weight percent, of 1 to 15% SiO₂, 25 to 41% B₂ O₃, 0 to 8% Al₂ O₃, 10 to 41% La₂ O₃, 2 to 25% Gd₂ O₃, 0 to 10% Y₂ O₃, the sum of the ZnO + CaO being 2 to 20%, the sum of the CdO + PbO + BaO + SrO + MgO being 0 to 10% the sum of the Ta₂ O₅ + Nb₂ O₅ + WO₃ + TiO₂ being 0 to 3%, and 0 to 5% ZrO₂ ; and

a thorium oxide-free optical glass wherein the glass has the composition, in weight percent, of 1 to 20% SiO, 25 to 41% B₂ O₃, 0 to 10% Al₂ O₃, 2 to 25% La₂ O₃, 2 to 15% Gd₂ O₃, 0 to 10% Y₂ O₃, the sum of the BaO + SrO + CaO + MgO being 10 to 50%, the sum of ZnO + CdO + PbO being 0 to 10%, the sum of the Ta₂ O₅ + Nb₂ O₅ + WO₃ + TiO₂ being 0 to 3%, and 0 to 8% ZrO₂.

The optical glasses of this invention are stable to devitrification even if they do not contain ThO₂ which is poisonous to humans and are colorless and transparent as well as excellent in chemical durability. In particular, the optical glass of this invention has optical constants of 1.72-1.85 for n_(d) and 50-27 for ν_(d) or of 1.68-1.86 for n_(d) and 53-30 for ν_(d) without the necessity of CdO.

While the invention has been described in detail and in terms of specific embodiments, it will be apparent that modifications and changes can be made therein without departing from the spirit and scope thereof. 

What is claimed is:
 1. A thorium oxide-free glass having a liquidus temperature not greater than 1085°C and having optical constants of 1.62-1.91 for n_(d) and 59-27 for νd consisting essentially of a composition in weight percent of 18 to 60% B₂ O₃, + SiO₂, a quantity of B₂ O₃ being present and the content of said SiO₂ being 0 to 23%, 0 to 50% La₂ O₃, 2to 50% Gd₂ O₃, 2 to 30% Ta₂ O₅, 0 to 50% ZnO, 0 to 53% CdO, 0 to 60% PbO, 0 to 50% BaO, 0 to 45% SrO, 0 to 30% CaO, 0 to 20% MgO, the sum of said ZnO + CdO + PbO + BaO + SrO + CaO + MgO being 2 to 60%, 0 to 16% Al₂ O₃, 0 to 9% ZrO₂, 0 to 10% TiO₂, 0 to 26% Nb₂ O₅, 0 to 15% WO₃, and 0 to 18% Y₂ O₃.
 2. The thorium oxide-free optical glass as described in claim 1, wherein said glass has the composition, in weight percent, of 18 to 40% B₂ O₃, 10 to 45% La₂ O₃, 2 to 35% Gd₂ O₃, 2 to 25% Ta₂ O₅, 0 to 25% ZnO, 0 to 30% CdO, the sum of said ZnO + CdO being 2 to 30%.
 3. The thorium oxide-free optical glass as described in claim 1, wherein said glass has the composition, in weight percent, of 22 to 40% B₂ O₃, 0 to 30% La₂ O₃, 2 to 30% Gd₂ O₃, 2 to 30% Ta₂ O₅, 25 to 40% ZnO, 0 to 45% PbO, the sum of said ZnO + PbO being 25 to 45%.
 4. The thorium oxide-free optical glass as described in Claim 1, wherein said glass has the composition, in weight percent, of 25 to 45% B₂ O₃, 0 to 40% La₂ O₃, 2 to 30% Gd₂ O₃, 2 to 20% Ta₂ O₅, 0 to 40% BaO, 0 to 35% SrO, 0 to 20% CaO, 0 to 20% MgO, the sum of said BaO + SrO + CaO + MgO being 2 to 40%.
 5. A thorium oxide-free optical glass having optical constants of 1.77-1.85 for n_(d) and 51-40 for νd consisting essentially of a composition in weight percent of 0 to 8% SiO₂, 21 to 30% B₂ O₃, 0 to 3% Al₂ O₃, 25 to 40% La₂ O₃, 2 to 25% Gd₂ O₃, 2 to 20% ZnO, the sum of said CdO + PbO + CaO + BaO + SrO + CaO + MgO being 0 to 10%, 0 to 6% ZrO₂, 2 to 15% Ta₂ O₅, 0 to 12% Nb₂ O₅, 0 to 6% WO₃, 0 to 3% TiO₂ and 0 to 10 Y₂ O₃.
 6. A thorium oxide-free optical glass having optical constants of 1.59-1.86 for n_(d) and 62-19 for ν d consisting essentially of a composition, in weight percent, of 24 to 70% B₂ O₃ + SiO₂, a quantity of B₂ O₃ being present and the content of said SiO₂ being 0 to 25%, 1 to 50% La₂ O₃, 2 to 50% Gd₂ O₃, 0 to 56% ZnO, 0 to 61% CdO, 0 to 66% PbO, 0 to 50% BaO, 0 to 40% SrO, 0 to 25% CaO, 0 to 20% MgO, the sum of said ZnO + CdO + PbO + BaO + SrO + CaO + MgO being 2 to 66%, 0 to 20% Al₂ O₃, 0 to 10% ZrO₂, 0 to 10% TiO₂, 0 to 15% Nb₂ O₅, 0 to 10% WO₃, and 0 to 20% Y₂ O₃.
 7. The thorium oxide-free optical glass as described in claim 6, wherein said glass has the composition, in weight percent, of 24 to 45% B₂ O₃, 1 to 45% La₂ O₃, 2 to 30% Gd₂ O₃, 0 to 25% ZnO, 0 to 30% CdO, the sum of said ZnO + CdO being 2 to 30%.
 8. The thorium oxide-free optical glass as described in claim 6, wherein said glass has the composition, in weight percent, of 25 to 45% B₂ O₃, 1 to 35% La₂ O₃, 2 to 25% Gd₂ O₃, 25 to 40% ZnO, 0 to 45% PbO, the sum of said ZnO + PbO being 0 to 45%.
 9. The thorium oxide-free optical glass as described in claim 6, wherein said glass has the composition, in weight percent, of 30 to 50% B₂ O₃, 1 to 45% La₂ O₃ , 2 to 30% Gd₂ O₃, 0 to 40% BaO, 0 to 35% SrO, 0 to 25% CaO, 0 to 20% MgO, the sum of said BaO + SrO + CaO + MgO being 2 to 40%.
 10. The thorium oxide-free optical glass as described in claim 6, wherein said glass has the composition, in weight percent, of 1 to 10% SiO₂, 28 to 37% B₂ O₃, 0 to 3% Al₂ O₃, 27 to 45% La₂ O₃, 2 to 15% Gd₂ O₃, 0 to 10% Y₂ O₃, 2 to 15% ZnO, the sum of said CdO + PbO + BaO + SrO + CaO + MgO being 0 to 10%, the sum of said Nb₂ O₅ + WO₃ + TiO₂ being 0 to 3%, and 0 to 8% ZrO₂.
 11. The thorium oxide-free optical glass as described in claim 10, wherein said Y₂ O₃ is 2 to 10%.
 12. The thorium oxide-free optical glass as described in claim 6, wherein said glass has the composition, in weight percent, of 1 to 20% SiO₂, 25 to 41% B₂ O₃, 0 to 10% Al₂ O₃, 2 to 25% La₂ O₃, 2 to 15% Gd₂ O₃, 0 to 10% Y₂ O₃, the sum of said BaO + SrO + CaO + MgO being 10 to 50%, the sum of said ZnO + CdO + PbO being 0 to 10%, the sum of said Ta₂ O₅ + Nb₂ O₅ + WO₃ + TiO₂ being 0 to 3%, and 0 to 8% ZrO₂. 